The invention is based on a flat gas discharge lamp according to the preamble of claim 1.
These are, in particular, flat gas discharge lamps, also referred to in brief below as flat lamps, having dielectrically impeded electrodes, as they are known. In this case, the dielectrically impeded electrodes are typically implemented in the form of thin metallic electrode tracks which are arranged on the outer wall and/or inner wall of the discharge vessel. If all the electrodes are arranged on the inner wall, at least some of the electrodes must be completely covered with respect to the interior of the discharge vessel by a dielectric layer.
Flat lamps of this type are used, for example, for backlighting liquid crystal displays (LCD) or else for general lighting, decorative and advertising purposes.
Otherwise, the technology of flat gas discharge lamps for dielectrically impeded discharges is assumed here as the prior art. As an example, reference is additionally made to the specification WO98/43277, whose disclosure content with regard to the lamp technology of flat gas discharge lamps for dielectrically impeded discharges is hereby incorporated by reference.
Flat gas discharge lamps of the generic type typically have two discharge vessel walls which, at least in some areas and approximately, are planar and are adjacent and parallel to one another.
These two vessel walls, referred to below as the top plate and base plate for brevity, are usually connected to each other in a gastight manner by a frame and in this way form the discharge vessel. Alternatively, the base plate and/or top plate can be formed in such a way that when they are joined together a discharge vessel is already formed. For example, the base plate and/or top plate can be of trough-like form, for example as a result of deep-drawing a planar glass plate. In the case of very large-area flat lamps, the predominant proportion of the shaped base plate and top plate, respectively, is at least approximately planar in this case as well. In any case, a lamp of this type needs one or more supporting points, also referred to below as spacer elements, for the purpose of stabilization.
This is all the more true since a discharge lamp has a gas filling of defined composition and with a filling pressure and therefore has to be evacuated before being filled. Consequently, the discharge vessel must permanently withstand both vacuumxe2x80x94specifically during production of the lampxe2x80x94and the subsequent filling pressure which, in the case of lamps of this type, is usually less than atmospheric pressure, for example between 10 kPa and 20 kPa. This is achieved by means of the aforementioned spacer elements, which are arranged in a sufficient number and suitable position between the base plate and front plate of the discharge vessel. In this case, each spacer element touches two mutually opposite supporting surfaces on the two plates and in this way supports them mutually.
When positioning the spacer elements, account has primarily to be taken of the stability of the arrangement. In addition, it is necessary to take care that the discharge is not influenced, or at most, slightly. In this respect reference is made to the specification WO99/54916. The spacer elements used there consist of a dielectric material, for example a soft glass or a ceramic.
The disadvantage is that the spacers are imaged as relatively dark spots in the luminous front plate of the lamp. As a result, the homogeneity of the luminance of the lamp is impaired. This is unacceptable, in particular in the case of backlighting liquid crystal displays. For this reason, optical diffusers, for example diffuser films, are usually used between the flat lamp and the liquid crystal displays. However, diffuser films of this type have transmission losses, as a result of which the effective luminance decreases. The aim is, therefore, to manage with as few diffuser films as possible, or, ideally, to dispense with diffusers completely.
It is an object of the present invention to provide a flat gas discharge lamp having spacer elements according to the preamble of claim 1, in which the spacer elements impair the homogeneity of the luminance of the lamp as little as possible.
In a lamp having the features of the preamble of claim 1, this object is achieved by the features of the characterizing part of claim 1.
Particularly advantageous refinements are to be found in the dependent claims.
According to the invention, the at least one spacer element arranged between the base plate and top plate of the discharge vessel of a flat lamp is additionally configured as a dielectrically impeded electrode. In other words, a spacer element of this type not only performs a supporting function, as in the prior art, but also an electrode function in addition.
In this way, during the operation of the lamp, a discharge burns between the spacer element and an adjacent electrode of opposite polarity. This electrode can also be a further such spacer element with an additional electrode function according to the invention. For the action aimed at according to the invention, it is essential only that a discharge starts up directly on the spacer element or on each spacer element. As a result, the relevant spacer element, so to speak, specifically contributes actively to the generation of light. In this case, it has been shown that the spacer elements modified in this way themselves light up in a certain way, in each case such that the inhomogeneity in the luminance normally caused by the spacer elements can either be avoided virtually completely or at least can be reduced considerably.
A spacer element according to the invention has both a first dielectric component and, in addition, an electrically conductive second component. In this case, the second component can also extend along the entire longitudinal extent of the spacer element, but does not necessarily have to do so but can instead also be restricted to only a part. For the additional function as a dielectrically impeded electrode, it is merely essential for the second, that is to say electrically conductive, component to be separated from the interior of the discharge vessel by the first dielectric component. For the function as a supporting point, it is additionally necessary for the spacer elementxe2x80x94at least the dielectric componentxe2x80x94to extend from the base plate to the top plate.
For the second electrically conductive component, a round or flat wire, a strip-like film or the like is particularly suitable. In this case, the first component consists of an insulating material, for example a glass sheath, in which the wire is enclosed. The first dielectric component does not necessarily have to be in one piece or consist of a single material. With regard to the supporting function, it may be advantageous if the spacer element consists of at least two materials of different hardnesses. In this regard, reference is made to the specification DE 198 17 478 A1, whose disclosure content is hereby incorporated by reference.
Alternatively, the two components can also consist of a metal/glass composite. In this case, it is advantageous if the concentration of the metal powder increases from the outside to the inside.
The spacer elements modified in accordance with the invention can supplement the actual electrodes, that is to say be arranged in addition to the latter, or can replace these, at least partially or even completely. In addition, the modified spacer elements can be used on their own or else together with conventional spacer elements.
The modified spacer elements, that is to say the electrically conductive second components, are electrically conductively connected to the current feeds to the flat lamp or to the actual electrodes. Of course, each spacer must be connected only to one electrical polarity in order to develop the desired additional electrode function.
If the electrodes of the flat lamp are configured as electrode tracks which are arranged on the inner surface of at least one of the two vessel plates, it has proven to be beneficial to arrange the modified spacer elements directly on the electrode tracks.
In order to facilitate the ignition of a discharge between the spacer elements of different polarity modified in accordance with the invention, or in order to permit it at all, the mutual spacing of the electrically conductive components is preferably less than the mutual spacing of the corresponding electrode tracks.
The solution according to the invention has the advantage of an improved homogeneity, which also permits the omission of an optical diffuser, associated with cost savings and a low overall height. In addition, the discharge between the spacer elements modified in accordance with the invention is considerably further removed from the base plate than is the case in typical flat lamps with electrode tracks arranged on the inner side of the top plate. As a result, the discharge vessel is heated less, and a phosphor layer which may be applied to the base plate is damaged less.